When Mount St. Helens erupted, trapped gases caused the north side of the mountain to explode. Volcanic ash was ejected high into the atmosphere. - PowerPoint PPT Presentation

1 / 35
About This Presentation
Title:

When Mount St. Helens erupted, trapped gases caused the north side of the mountain to explode. Volcanic ash was ejected high into the atmosphere.

Description:

When Mount St. Helens erupted, trapped gases caused the north side of the mountain to explode. Volcanic ash was ejected high into the atmosphere. – PowerPoint PPT presentation

Number of Views:98
Avg rating:3.0/5.0
Slides: 36
Provided by: Nathan327
Category:

less

Transcript and Presenter's Notes

Title: When Mount St. Helens erupted, trapped gases caused the north side of the mountain to explode. Volcanic ash was ejected high into the atmosphere.


1
  • When Mount St. Helens erupted, trapped gases
    caused the north side of the mountain to explode.
    Volcanic ash was ejected high into the atmosphere.

2
  • A volcano is a mountain that forms when magma
    reaches the surface.
  • Volcanoes can result from several different
    geological processes and can take a variety of
    forms.

3
Formation of a Volcano
4
Formation of a Volcano
  • The process that leads to a volcanic eruption
    begins deep inside Earth.
  • Magma rises because it is less dense than the
    solid rock around and above it.

5
Formation of a Volcano
  • How a Volcano Erupts
  • Magma is under pressure and contains dissolved
    gases, including carbon dioxide and water vapor.
  • Lower pressure near the surface allows the gases
    in magma to expand rapidly.
  • An eruption occurs when the gases bubble out
    through a crack in the crust, propelling magma to
    the surface.

6
Formation of a Volcano
  • Structure of a Volcano
  • Before an eruption, magma often collects in a
    pocket called a magma chamber.
  • Magma slowly accumulates in the magma chamber
    until enough pressure builds up to start an
    eruption.
  • Then, magma rises to the surface in a narrow,
    vertical channel called a pipe.

7
Formation of a Volcano
  • An opening in the ground where magma escapes to
    the surface is called a vent.
  • Often there is one central vent at the top of a
    volcano. Sometimes there are other vents that
    open along a volcanos side.
  • At the top of the central vent in most volcanoes
    is a bowl-shaped pit called a crater.

8
Formation of a Volcano
  • After an eruption, a volcanos magma chamber and
    main vent may empty of magma, creating a hollow
    shell.
  • If this shell collapses inward, it creates a huge
    depression, called a caldera, at the top of the
    volcano.

9
Formation of a Volcano
  • When a volcanic mountain erupts, magma under
    pressure is forced upward from the magma chamber.
    Magma flows onto the surface as lava.

Crater
Vent
Magma chamber
Pipe
Lava
10
Quiet and Explosive Eruptions
11
Quiet and Explosive Eruptions
  • Magma can vary in viscosity, the resistance to
    flow.
  • Magma with high viscosity is thick and resists
    flowing. Magma with low viscosity is thin and
    flows easily.

12
Quiet and Explosive Eruptions
  • There are three main factors that determine the
    viscosity of magma temperature, water content,
    and silica content.
  • Higher temperatures lower the viscosity of magma,
    so it flows more easily.
  • Water in magma helps it flow more easily.
  • Magma that is high in silica has high viscosity.

13
Quiet and Explosive Eruptions
  • Quiet Eruptions
  • Volcanoes that have very hot, low-silica magma
    generally erupt quietly.
  • In a quiet eruption, lava erupts in a stream of
    low-viscosity lava, called a lava flow.
  • Lava flows from a quiet eruption can travel for
    great distances.

14
Quiet and Explosive Eruptions
  • Quiet eruptions produce two different kinds of
    lava chunks called aa, or smooth coils called
    pahoehoe.

pahoehoe
aa
15
Quiet and Explosive Eruptions
  • Mt. Kilauea in Hawaii erupts quietly, producing
    low-viscosity lava flows.

16
Quiet and Explosive Eruptions
  • Explosive Eruptions
  • High-silica magma produces explosive eruptions.
  • Thick magma can clog a volcanic pipe, causing
    enormous pressure to build up.
  • When the volcano finally explodes, lava and hot
    gases are hurled outward.

17
Location and Types of Volcanoes
18
Location and Types of Volcanoes
  • Volcanoes often form along a converging plate
    boundary where an oceanic plate is subducted into
    the mantle.
  • As it sinks through the mantle, the plate causes
    melting.
  • Magma forms and rises to the surface.

19
Location and Types of Volcanoes
  • Volcanoes also form along a diverging plate
    boundary where magma rises to fill the gap
    between two separating plates.
  • Some volcanoes occur at hot spots, regions where
    hot rock extends from deep within the mantle to
    the surface.

20
Location and Types of Volcanoes
  • Except for hotspot volcanoes, most of the worlds
    volcanoes form near plate boundaries.

21
Location and Types of Volcanoes
22
Location and Types of Volcanoes
  • Different types of volcanic eruptions produce
    different types of volcanoes.
  • Each type is named for its shape or interior
    structure.

23
Location and Types of Volcanoes
  • A quiet eruption of low-viscosity lava produces a
    wide, flat volcano called a shield volcano.
  • If an eruption is entirely ash and cinders, the
    result will be a small, steep-sided volcano
    called a cinder cone.
  • A volcano that forms from explosive eruptions
    that produce a combination of lava and ash is
    called a composite volcano.

24
Other Igneous Features
25
Other Igneous Features
  • Sometimes magma does not reach the surface, but
    cools and hardens in the crust.
  • This magma forms intrusive igneous rock that may
    eventually be forced upward and exposed at
    Earths surface.
  • Lava plateaus are features formed of extrusive
    igneous rock.

26
Other Igneous Features
  • A batholith is the largest type of intrusive
    igneous rock mass.
  • Magma sometimes squeezes into a crack between
    layers of rock.
  • If the crack is parallel to existing rock layers,
    the magma hardens into a structure called a sill.
  • If the crack cuts across rock layers, the
    hardened magma forms a dike.
  • When magma hardens in a volcanos pipe, a
    structure called a volcanic neck may form.

27
Other Igneous Features
  • Ship Rock in New Mexico is a volcanic neck. It
    formed when the soft rock around a volcanos pipe
    wore away, revealing hard, igneous rock. The long
    ridge extending from the volcanic neck is a dike.

28
Assessment Questions
  • An opening in the ground where magma escapes to
    the surface is called a
  • vent.
  • crater.
  • pipe.
  • magma chamber.

29
Assessment Questions
  • An opening in the ground where magma escapes to
    the surface is called a
  • vent.
  • crater.
  • pipe.
  • magma chamber.ANS A

30
Assessment Questions
  • A quiet eruption occurs when a volcano is filled
    with
  • very hot, low-silica magma.
  • high-silica magma.
  • cinders and lava bombs.
  • water

31
Assessment Questions
  • A quiet eruption occurs when a volcano is filled
    with
  • very hot, low-silica magma.
  • high-silica magma.
  • cinders and lava bombs.
  • water.ANS A

32
Assessment Questions
  • Short, isolated chains of volcanoes, such as the
    Hawaiian Islands, occur at
  • divergent boundaries.
  • convergent boundaries.
  • hot spots.
  • rift valleys.

33
Assessment Questions
  • Short, isolated chains of volcanoes, such as the
    Hawaiian Islands, occur at
  • divergent boundaries.
  • convergent boundaries.
  • hot spots.
  • rift valleys.ANS C

34
Assessment Questions
  • What type of structure forms when magma hardens
    in a volcanos pipe?
  • a batholith
  • a sill
  • a dike
  • a volcanic neck

35
Assessment Questions
  • What type of structure forms when magma hardens
    in a volcanos pipe?
  • a batholith
  • a sill
  • a dike
  • a volcanic neckANS D
Write a Comment
User Comments (0)
About PowerShow.com